Odor localization in structural interhemispheric deficits

Abstract

Contrary to all other sensory systems, olfactory information is processed predominantly ipsilaterally. Furthermore, odor localization, based on inter-nostril differences, is usually not possible under controlled conditions. These two observations suggest information exchange between both cerebral hemispheres in the olfactory system, although the exact anatomical substrate remains unknown. This study aimed to identify the anatomical structures necessary for odor localization, with a particular focus on the role of interhemispheric communication. We assessed the ability to localize pure olfactory and mixed olfactory/trigeminal stimuli in 6 participants with structural interhemispheric deficits (including surgical transection or agenesis of the corpus callosum (CC) and agenesis of the CC and anterior commissure (AC, one case)) and compared their performance to 46 healthy controls. Of the six participants with structural interhemispheric deficits, three were unable to localize either stimulus. Two participants performed significantly better than chance for both pure and mixed stimuli, while one participant exhibited the typical localization pattern observed in most controls-accurate localization of the mixed olfactory/trigeminal stimulus but inability to localize the pure olfactory stimulus. Our results suggest that localization of chemosensory stimuli relies, at least in part, on CC, highlighting its role in interhemispheric communication for olfactory processing. The varying odor localization performance observed in participants with agenesis of CC indicates that compensatory mechanisms may be promoted in some cases, potentially preserving normal localization functions despite the absence of major commissural pathways.

Keywords: commissural deficits; lateralization; olfaction; structural connectivity; trigeminal system.

Fig. 1.

Performance patterns of odor localization for phenyl ethanol and eucalyptol for control participants (dots) and patients (squares). The green box highlights scores over chance for eucalyptol (>26) and by chance for phenyl ethanol (between 14 and 26), which are the expected values for normal localization performance. Anything outside the green box is considered as abnormal performance. All patients but one performed outside normal values.

Fig. 2.

Analysis of sensitivity (d′) of odor localization for phenyl ethanol and eucalyptol for control participants (left) and patients (right). Mann–Whitney U test revealed a significant effect of stimuli (control: P = 2.186e-12; patient: P = 0.026) but did no difference between groups (PEA: P = 0.774; EUC: P = 0.223).